Process for producing tungsten-alloy emitter type electrode

ABSTRACT

AN EMITTER TYPE ELECTRODE COMPRISING A HEAD CONSISTING ESSENTIALLY OF A MOLDABLE EMISSIVE ELECTRODE MATERIAL, TUNGSTEN, AND FROM ABOUT 0.2% TO ABOUT 7% OF A GROUP V111 METAL ADDITIVE THAT LOWERS THE SINTERING TEMPERATURE OF TUNGSTEN AT LEAST ABOUT 100* C. AND INTEGRAL WITH SAID HEAD AND EXTENDING THEREFROM, AN ELECTRICALLY CONDUCTIVE TENGSTEN ALLOY LEAD CONSISTING ESSENTIALLY OF TENGSTEN AND FROM ABOUT 1% TO ABOUT 30% RHENIUM. IN THE PROCESS FRO PREPARING EMITTER-TYPE ELECTRODES HAVING AN INTEGRAL HEAD AND LEAD BY MOLDING, PRESSING AND SINTERING THE IMPROVEMENT COMPRISING ADDING TO THE MOLDABLE, EMISSIBE ELECTRODE MATERIAL AND TUNGSTEN, THAT FORMS THE HEAD, PRIOR TO PRESSING AND SINTERING, FROM ABOUT 0.2% TO ABOUT 7% OF A GROUP CIIIMETAL ADDITIVE AND INSERTING INTO THE HEAD PRIOR TO PRESSING AND SINTERING A TUGSTEN ALLOY LEAD CONSISTING ESSENTIALLY OF TUNGSTEN AND FROM ABOUT 1% BY WEIGHT TO ABOUT 30% OF RHENIUM.

United States Patent O 3,745,000 PROCESS FOR PRODUCING TUNGSTEN-ALLOYEMIITER TYPE ELECTRODE Richard F. Cheney, Towanda, Pa., and Nicholas W.

Rench, Troy, N.Y., assignors to GTE Sylvania Incorporated No Drawing.Original application Oct. 22, 1970, Ser. No. 83,197, now Patent No.3,684,912. Divided and this application Jan. 14, 1972, Ser. No. 217,987

Int. Cl. B22f 7/08 US. Cl. 75208 R 5 Claims ABSTRACT OF THE DISCLOSUREAn emitter type electrode comprising a head consisting essentially of amoldable emissive electrode material, tungsten, and from about 0.2% toabout 7% of a Group VIII metal additive that lowers the sinteringtemperature of tungsten at least about 100 C. and integral with saidhead and extending therefrom, an electrically conductive tungsten alloylead consisting essentially of tungsten and from about 1% to about 30%rhenium. In the process for preparing emitter-type electrodes having anintegral head and lead by molding, pressing and sintering theimprovement comprising adding to the moldable, emissive electrodematerial and tungsten, that forms the head, prior to pressing andsintering, from about 0.2% to about 7% of a Group VIII metal additiveand inserting into the head prior to pressing and sintering a tungstenalloy lead consisting essentially of tungsten and from about 1% byweight to about 30% of rhenium.

CROSS-REFERENCE TO RELATED APPLICATION This application is a division ofSer. No. 83,197, filed Oct. 22, 1970 and assigned to the assignee of thepresent invention. This application is now US. Pat. 3,684,912.

BACKGROUND OF THE INVENTION This invention relates to emitter-typeelectrodes. More particularly, it relates to emitter-type electrodesthat have an integral head and lead wherein the lead has many of thedesirable properties of tungsten and which lead is resistant toembrittlement.

This invention relates broadly to the art of producing emiter-typeelectrode structures, which also may be de signated as emissive (i.e.,electron-emissive) electrode structures. More particularly, it isconcerned with the fabrication of an electrode that is especiallyadapted for use as a high-voltage cold electrode (anode or cathode)element in such devices as, for example, spark-gap assemblies, flashlamps, flash tubes and the like. The scope of the invention includesboth article and method features.

Emitter-type electrode heads heretofore were made from powderedmaterials by techniques that involve pressing and sintering. Typically,the head consists of an emissive (including potentially emissive)material that is dispersed in a porous refractory metal matrix, e.g.,tungsten, molybdenum, or mixtures of either or both with otherrefractory metals; or, alternatively, such electrodes can be describedas consisting essentially of a porous refractory metal matriximpregnated or mixed with the emissive material. The emissive materialcan be, for instance, a compound of an alkaline-earth metal, e.g., anoxide, carbonate, aluminate or orthosilicate either alone or in certaincombinations.

It will be understood, of course, by those skilled in the art that byemitter-type electrode or electrode structure is meant an electrode thatfunctions during use, more particularly, under high-voltage application,to emit electrons. By cold electrode or electrode structure is meant anemitter-type electrode device having an emissive surice face that, inuse, emits electrons only under high voltage as contrasted with anelectrode or electrode structure that, in use, is continuously heated bya filamentary conductor. An example of the latter is, for example, adispenser cathode.

Sintered electrode heads, specifically sintered cathodes, usually havethe general form of a disc. The emitting surface can be, for example,flat, concave, convex or conical. The opposite side is usually flat. Oneend of a lead (i.e., lead-in conductor) consisting of an electricallyconducting rod or wire, having a diameter less than that of theelectrode, can be attached to the fiat surface of the electrode bybrazing. Although tungsten has many desirable properties for leads andthe matrix for the head, brazing is difiicult.

Because of the close physical relationship between the lead and theelectron-emissive surface, this brazing step is a very troublesomeoperation particularly when tungsten is used. Furthermore, the brazingproblems increase as the percentage of alkaline-earth emissive materialin the electrode composition (which may be from about 5% to about 20% byweight thereof) is increased to a percentage near to and above the usualamount of about 10 weight percent of the aforesaid composition. An evenmore serious problem is often encountered during operation of thearticle or device, containing the electrode element, as a result ofbleeding of the braze material onto the emissive surface. Such bleedingcan cause the device to operate improperly.

To overcome the problems associated with brazing the head and lead, atechnique was developed for using metal powders, casting, sintering themto form the head and stem as one integral part. This technique isdisclosed in US. Pat. 3,489,554.

Although the problems with brazing were overcome when tungsten was usedas the material for the matrix for the head and the lead, additionalproblems resulted. These problems resulted partially from the hightemperatures used to sinter tungsten and resulted in some cracking ofthe parts. Use of metal additives in the head to lower the sinteringtemperature of tungsten in the head resulted in an inducedrecrystallization and the resultant embrittlement of the tungsten leadfrequently results so thatrelatively slight pressures to the lead causesthe part to break.

It is believed, therefore, that an emissive electrode that is notbrazed, that has a lead that has the desired properties of tungsten andthat can be sintered at a lower temperature or for a shorter timewithout embrittlement is an advancement in the art.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of this inventionto provide an emissive electrode that overcomes the problem of brazingbut is structurally more sound than prior electrodes having tungstenleads.

It is a further object of this invention to provide an improved processfor providing emissive electrodes having an integral head and lead.

In accordance with one aspect of this invention, there is provided anemissive electrode comprising a head consisting essentially of anemissive material, tungsten, and from about 0.2% to about 7.0% of aGroup VIII metal additive that lowers the sintering temperature oftungsten at least about C. and integral with and extending from the heada lead consisting essentially of a tungsten-base alloy consistingessentially of tungsten and from about 1% to about 30% of rhenium basedupon the weight of the tungsten.

In accordance with an additional aspect of this invention there isprovided an improvement to the process for manufacturing parts having ahead with a tungsten matrix and a tungsten alloy lead. The improvementcomprises incorporating in the material used to form the head, prior topressing and sintering, tungsten and about 0.5% to about 7.0% of anadditive that lowers the sintering temperature of tungsten at least 100C. and using a tungsten alloy stem consisting essentially of tungstenand from about 1% to about 30% of rhenium.

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above description of some of the aspects of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Briefly described, the articleis constructed by pressing a powdered, moldable, emissive, electrodehead material hereinafter described about the end of an electricallyconducting lead of a particular tungsten alloy hereinafter described.Molding pressures can range from about 10,000 to about 80,000 p.s.i.g.or more as can be used depending upon the particular materials beingpressed and structure being fabricated. During subsequent sintering ofthe pressed assembly, the electrode or head and the lead or stud becomefixedly attached or united to each other thereby forming an integralstructure.

The electrically conducting lead (or stem or stud as it also can bedesignated) comprises or consists essentially of a particular tungstenalloy as hereinafter described.

In producing the composite structures of this invention, the head isformed by (a) a powdered emissive material (examples of which have beengiven hereinbefore,)

(b) a tungsten metal powder generally referred to as the matrix isemployed in the approximate weight percentages of from 520% of theformer to from 9580% of the latter, and

(c) from about 0.2 to about 7% by weight of the tungsten of a Group VIIImetal additive that lowers the sintering temperature of tungsten atleast about 100 C.

These ingredients together with a small amount (e.g., from about 0.5 toabout 3% of the weight of the primary components) of a temporary organicbinder, for instance from 1 to 2 Weight percent (on this same basis) ofa wax or wax-like substance of natural or synthetic origin, are firstthoroughly mixed together. When the organic binder is, for example,parafiin wax (in slivered or other suitable form) sufiicient heat isgenerated during mixing to melt the paraffin thereby facilitating itsmore uniform dispersion throughout the mix. With higher melting waxes orWax-like substances it may be necessary to heat the mixture whileadmixing in order to efiectively disperse the binder.

Any desired configuration of the head can be pressed, e.g., flat,concave, convex, pointed or conical, etc. Likewise the diameters andlengths of the studs can be varied as desired or as conditions mayrequire.

Although in general any Group VIII metal that lowers the sinteringtemperature of tungsten at least 100 C. such as iron, nickel, cobalt,rodium, palladium and the like can be used, iron, cobalt and nickel arepreferred and of these nickel is especially preferred. Preferred amountsof the metal additive are from about 0.25% to about 1% by weight oftungsten.

A tungsten alloy lead consisting essentially of tungsten and from about1% to about 30% of rhenium is used. The use of rhenium apparentlycreates a resistance to the embrittlement normally induced by themigration of the Group VIII metal additive to the usual tungsten lead.In this manner, the benefits of the lower sintering tem peratures areattained without the embrittlement experienced heretofore. The tungstenalloy containing rhenium in the amounts herein specified has thedesirable electrical and refractory e a p perties of t g F om abou 1.5%to about 5.0% by weight of rhenium is preferred with about 3% to about3.5% being especially preferred.

After pressing the moldable electrode material onto one end of the lead,the resulting composite is dewaxed, fired (i.e., sintered) in a reducingatmosphere, usually hydrogen. The temperatures employed during thesintering step are below 1700" C. but will be dependent upon theparticular metal additives used in the pressed electrode composition ofwhich the head is formed.

Nickel, for example, when used within the 0.2% to 7% range can lower thetemperature to about 1550 C. and reduce the sintering to about /2 hour.When tungsten alone is the matrix a temperature of about 1800 C. forabout 10 hours is required. The improvement of this invention offersappreciable increases in production throughput. In most instances, it isdesired to use amounts and types of additives that after dewaxing at 800C. for 1 /2 hours an electrode is formed below 1600 C. in less than 1hour.

In order to more fully illustrate the subject invention, the followingdetailed example is presented. All parts, proportions and percentagesare by Weight unless otherwise indicated.

Example (1) To a high intensity mixer are charged and therein thoroughlyadmixed: 450 parts tungsten powder having an average diameter of about 4microns; 50 parts powdered barium aluminate (ZOO-mesh particle size,U.S. Standard sieve series), and 7.5 parts of slivered paraflin wax andabout 5 parts of nickel powder.

(2) The resulting substantially homogeneous admixture is removed fromthe mixer and allowed to become harder.

(3) The mixture from step (2) is forced through a 20-mesh screen (U.S.Standard sieve series) to provide a powder that flows freely.

(4) The powder is allowed to flow into the cavity of a double-acting diemounted in a rotary mechanical press.

(5) Simultaneously with step (4) a tungsten alloy lead in the form of a1.050 long rod containing tungsten and 3.25% by weight of rhenium isplaced in the upper punch or plunger of the press.

(6) The press is actuated such that the powder is pressed about the endof the lead at a pressure of about 40,000 p.s.i.g.

(7) After the part has been ejected from the press, it is subjected to asintering operation. A satisfactory procedure consists of the followingsintering and finishing steps:

(a) Place the pressed part in ZrO- sand contained in a molybdenum tray.

(b) Dewax by heating to about 800 C. for about 1.5

hours to drive off volatile materials.

(c) Stoke the tray into the hot zone of a furnace operating at about1550 C. under an atmosphere of flowing (ca. 30-40 c.f.h.) hydrogen.After the part reaches the temperature of the furnace, it is maintainedat that temperature for about one-half hour.

(d) Stoke the tray into the cool zone.

(e) Hold the tray in the cool zone for about one hour.

(f) Remove the tray from the cool zone and remove the finished part fromthe tray.

(g) The collected, sintered parts are tumbled in methanol while incontact with polishing stones for about 1 hour.

The resulting head can not be broken from the lead using fingerpressure. It requires the use of two pairs of pliers, one gripping thehead while the other gripping the lead, to break the cathode.

A composite using a tungsten lead without rhenium is also prepared. Thehead breaks from the lead by applying finger pressure to the end of thelead While grasping the h ad in he other ha d.

Substantially similar results are achieved when other Group VIIIelements are substituted for the nickel in substantially similaramounts. For example, use of about 1% iron in the lead lowers thesintering temperature to about 1550 C.

Below about 0.2% by weight of the Group VIII metal additive is generallyineffective. Greater than about 7% by weight does not result in anyappreciable benefits and can alter the properties of the partsufiiciently to create some problems, therefore, are not recommended.

While there has been shown and described what are at present consideredthe preferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims.

What is claimed is:

1. In a process for producing an emitter type electrode having anintegral head and lead from powder head materials by molding, pressingand sintering the improvement comprising (a) forming a head materialconsisting essentially of a powdered moldable emissive electrodematerial, tungsten and from about 0.2% to about 7% by weight, based uponsaid tungsten, of a Group VIII metal additive that lowers the sinteringtemperature of tungsten at least 100 C., and (b) inserting into saidhead material, prior to pressing and sintering, an electricallyconductive tungsten alloy lead consisting essentially of tungsten andfrom about 1% to about 30% by weight of rhenium.

2. An improvement according to claim 1 wherein said metal additive isselected from the group consisting of iron, cobalt, and nickel.

3. An improvement according to claim 2 wherein said metal additive isnickel.

4. An improvement according to claim 3 wherein sa1d nickel is from about0.5% to about 4% by weight and said rhenium is from about 1.5% to about5.0%.

5. An improvement according to claim 4 wherein after pressing, said headand lead are heated at 800 C. for about 1.5 hours and sintered at belowabout 1600 C. for less than 1 hour.

References Cited UNITED STATES PATENTS 3,489,554 1/1970 Waldo 29182.3 X3,443,143 5/1969 Koo 29--182.7 X 2,488,731 11/1949 Lambert et a1. 2 9-1823 2,557,372 6/1951 Cerulli et al. 29l82.3 X 3,320,058 5/1967 Krock eta1. 75-208 R 3,372,024 3/1968 Parikh 75-200 X OTHER REFERENCES Toth etal., I. Less-Common Metals, 12, 353-365 (1967).

CARL D. QUARFORTH, Primary Examiner E. A. MILLER, Assistant Examiner US.Cl. X.R. 29182.3; 75200

